There are many conventional heat transfer processes which act to transfer thermal energy to or from an object through physical contact with a heat transfer liquid which is either at a temperature hotter or colder than the object. A number of organic solvents have been used as such heat transfer liquids, for example, Dowtherm for high temperature heat transfer processes and low molecular weight alcohols, ketones and halogenated hydrocarbons for low temperature heat transfer processes.
Low temperature heat transfer processes continue to have difficulties caused by the volatility, toxicity, flammability, foaming or low temperature viscosity changes of the conventional low temperature organic heat transfer liquids. Such conventional low temperature heat transfer liquids exhibit considerable viscosity change or freezing and foaming as their temperature is reduced. Furthermore, the freezing and boiling points of these heat transfer liquids necessarily restrict the operational temperature range of the heat transfer processes in which they are used.
Heat transfer liquids which might otherwise be useful may exhibit too high a freezing point or too low a boiling point, or both, to be efficiently employed in low temperature heat transfer processes. Specifically, the freezing or gelation of a low temperature heat transfer liquid will likely lead to a significant reduction in the efficiency of the thermal energy transfer. This reduction in efficiency will result from significant viscosity increases as well as clogging of transfer lines or other parts of the heat transfer apparatus which will act to interrupt or impede the circulation of the heat transfer liquid within the heat transfer process equipment.
Also, some conventional low temperature heat transfer liquids such as acetone, absorb moisture present in their surroundings. Thus, heat transfer processes employing such fluids may be adversely affected by a rise in the freezing point temperature due to the absorption of moisture. Accordingly, the absorbed moisture (water) may constitute a disadvantage for low temperature heat transfer processes.
One class of heat transfer liquids known for being beneficial for lower temperature heat transfer processes (between 0.degree. F. and -142.degree. F.) are certain chemical compounds of the class of monocyclic terpenes. U.S. Pat. No. 3,597,355 (Hsu) describes the use of monocyclic terpenes as a class, and d-limonene in particular, as being useful for low temperature applications employing heat transfer liquids. The class of monocyclic terpenes is described as consisting of limonene, dipentene, terpinolene, .alpha., .beta. and .gamma. terpinene, among others. The Hsu patent describes d-limonene as particularly preferred because of its characteristic properties. However, the Hsu patent does not describe the benefits of combining a terpene with an alkylbenzene to produce the composite heat transfer fluid compositions of the invention, which exhibit improved low temperature operating characteristics.
Monocyclic terpenes are chemical compounds conventionally used as solvents, and also for a variety of other purposes such as the flavoring of foodstuffs. Monocyclic terpenes are described in the Hsu patent as being particularly useful in heat transfer processes which employ a heat transfer liquid as a means by which the heat is transferred, but only with an example using d-limonene. Such a heat transfer process might involve the circulation, by means of a pump or convection, in a conduit system in heat exchange contact with an apparatus from which heat is to be removed. The heat transfer liquid circulated is maintained at a temperature lower than that of the apparatus to be cooled by a suitable cooling mechanism.
The Hsu patent describes monocyclic terpenes as exhibiting relatively little viscosity change over the entire liquid phase temperature range, and thus, can advantageously retain excellent fluidity even at low temperatures which are slightly above their respective freezing points. Additionally, monocyclic terpenes have low surface tensions and display excellent wetting of metallic and non-metallic surfaces, properties which enhance heat transfer efficiency and minimize ice formation on such surfaces.
The specific teaching of the Hsu patent is the use of single monocyclic terpenes for use as heat transfer liquids. Since the grant of the Hsu patent, the cost of orange oil and commercially available limonene (d- or l-) has increased significantly. Furthermore, according to the Hsu patent, naturally occurring limonene is suitable for use as a heat exchange liquid without further refinement. The Hsu patent continues by stating that it is usually desirable to subject naturally occurring limonene to further distillation to provide relatively pure d-limonene for use as a heat exchange liquid. Relatively pure d-limonene, however, exhibits a reduction in heat exchange efficiency at temperatures below -120.degree. F. At such temperatures the viscosity of relatively pure d-limonene increases significantly. At temperatures below -140.degree. F., relatively pure d-limonene begins to gel. Such viscosity increase and gelation effectively limits the use of relatively pure d-limonene to heat transfer processes that operate at temperatures above -120.degree. F.
What is needed are heat transfer fluid compositions with improved low temperature operating characteristics. Specifically, what is needed are heat transfer fluid compositions which remain in the liquid phase at temperatures from about 0.degree. F. to below -120.degree. F., preferably from about 0.degree. F. to about -175.degree. F.